While a number of recent efforts are being made to achieve a finer pattern rule in the drive for higher integration and operating speeds in LSI devices, the commonly used light exposure lithography is approaching the essential limit of resolution determined by the light source wavelength.
As the light source used in the lithography for resist pattern formation, g-line (436 nm) or i-line (365 nm) from a mercury lamp has been widely used. One means believed effective for further reducing the feature size is to reduce the wavelength of exposure light. For the mass production process of 64 M-bit DRAM, the exposure light source of i-line (365 nm) was replaced by a KrF excimer laser having a shorter wavelength of 248 nm. However, for the fabrication of DRAM with a degree of integration of 1 G or more requiring a finer patterning technology (processing feature size 0.13 μm or less), a shorter wavelength light source is required, and in particular, photolithography using ArF excimer laser light (193 nm) is now under investigation.
At the initial stage of KrF lithography, steppers having achromatic lenses or catoptric systems combined with broadband light were developed. However, since the precision of achromatic lenses or aspherical catoptric systems was insufficient, a combination of narrow band laser light with diotric single lenses became the main stream. In general, it is a well-known phenomenon for single wavelength exposure that incident light interferes with reflected light from the substrate to generate standing waves. A so-called halation phenomenon that light is collected or scattered by irregularities on the substrate is also known. Both the standing waves and the halation induce dimensional changes of pattern linewidth or the like, shape collapse or the like. The use of coherent monochromatic light, combined with a reduction of wavelength, further amplifies the standing waves and halation. Then as a method of suppressing halation or standing waves, a method of adding a light absorber to resist and a method of laying an antireflective film on the resist upper surface or on the substrate surface were proposed. However, the method of adding a light absorber gives rise to the problem that the resist pattern shape is tapered. With the recent progress toward shorter wavelengths and smaller feature sizes, the influence of standing waves and halation on pattern dimensional changes becomes more serious beyond the level that can be managed by the addition of a light absorber.
In principle, an overlying transmission type antireflection coating (ARC) is effective only for reducing standing waves, but not for halation. Since the refractive index of an overlying ARC to completely offset standing waves is ideally the square root of a refractive index of resist, an ideal value of 1.34 is given to ARC for combination with the polyhydroxystyrene-base resist for use in KrF lithography having a refractive index of 1.8. An ideal value of 1.27 is given to ARC for combination with the alicyclic acrylic resist for use in ArF lithography having a refractive index of 1.6. Materials having such a low refractive index are limited to perfluoro materials. However, the overlying ARC must be made of water-soluble materials because it is advantageous from the process aspect that the overlying ARC is strippable during alkaline development. If hydrophilic substituent groups are introduced into a highly hydrophobic perfluoro material to tailor it to be water soluble, the refractive index of the material is increased so that the ideal value is increased to about 1.42 for KrF and to about 1.5 for ArF. Then, if patterning to a size of 0.20 μm or less is done by KrF lithography, a mere combination of a light absorber with an overlying ARC fails to suppress the influence of standing waves. In the ArF lithography, the effect of an overlying ARC is almost unexpectable for the above-described reason. In the KrF lithography as well, it will become necessary to lay an antireflection film below the resist as a future reduction of linewidth imposes more severe management of linewidth.
In the case of an antireflective film underlying the resist, when a high reflection substrate such as polysilicon or aluminum underlies, a material having an optimum refractive index (n value) and absorption coefficient (k value) is designed to an appropriate film thickness, whereby the reflectance from the substrate can be reduced to below 1%, achieving a significant antireflection effect. In an example wherein the wavelength is 193 nm and a resist has a refractive index of 1.7, if the underlying ARC has a refractive index (real part of complex refractive index) n of 1.5, an extinction coefficient (imaginary part of complex refractive index) k of 0.5, and a thickness of 42 nm, then the reflectance becomes below 0.5%. However, if the substrate has steps, the ARC largely varies its thickness at the steps. The antireflection effect of the underlying film utilizes not only light absorption, but also the interference effect. The first base of 40 to 45 nm having the enhanced interference effect has an accordingly enhanced antireflection effect, but the reflectance largely varies with a change of film thickness. JP-A 10-69072 discloses a high conformity antireflective film-forming material in which the molecular weight of a base resin is increased to minimize the variation of film thickness at steps. As the molecular weight of a base resin increases, there arise problems that more pinholes generate after spin coating, filtration becomes difficult, a viscosity change with the passage of time leads to a variation of film thickness, and crystals precipitate at the nozzle tip. The conformal behavior is developed only at relatively low steps.
Another probable method uses a film thickness of at least the third base (i.e., of at least 170 nm) where the variation of reflectance due to a film thickness variation is relatively small. As long as the k value is in a range of 0.2 to 0.3 and the film thickness is at least 170 nm, the variation of reflectance due to a film thickness variation is small and the reflectance is kept below 1.5%.
In the event the underlay below the antireflective film is a transparent film like an oxide or nitride film and steps exist below that transparent film, the thickness of the transparent film varies even if the surface of the transparent film is planarized as by chemical mechanical polishing (CMP). In this event, it is possible to make the thickness of the overlying antireflective film uniform. If the thickness of a transparent film underlying the antireflective film varies, the thickness of the minimum reflective film is shifted by the thickness of the transparent film at a period of λ/2n (wherein λ is an exposure wavelength and n is a refractive index of the transparent film at that wavelength). Even if the thickness of the antireflective film is set equal to the thickness (5 nm) of the minimum reflective film when the underlay is a reflective film, there develop some areas having an increased reflectance due to thickness variations of the transparent film. In this regard, the thickness of the antireflective film must be at least 170 nm, as in the above-mentioned event, for stabilizing the reflectance relative to thickness variations of the underlying transparent film as well.
The materials of which the antireflective film is made are generally divided into inorganic and organic materials. A typical inorganic material is a SiON film. This has the advantages that it can be formed by CVD of a gas mixture of silane and ammonia, and the etching load on resist is light due to a high selective ratio of etching relative to resist, but the range of application is restricted because of difficulty of peeling. Because of a nitrogen atom-containing basic substrate, another drawback arises that it is susceptible to footing in the case of positive resist and an undercut profile in the case of negative resist.
The organic material has the advantages that spin coating is possible without a need for a special equipment as needed for CVD and sputtering, peeling is possible like resist, no footing occurs, the shape is obedient, and adhesion to resist is good. Thus a number of antireflective films based on organic materials have been proposed. For example, JP-B 7-69611 describes a composition comprising a condensate of a diphenylamine derivative with a formaldehyde-modified melamine resin, an alkali-soluble resin, and a light-absorbing agent. However, since most light-absorbing agents have aromatic groups or double bonds, the addition of a light-absorbing agent undesirably increases dry etching resistance and rather reduces a selective ratio of dry etching relative to the resist. As the feature size becomes finer, the drive toward resist film slimming is accelerated. In the ArF exposure lithography of the next generation, acrylic or alicyclic polymers are used as the resist material, indicating that the etching resistance of the resist becomes poor. A further consideration is the problem that the thickness of antireflective film must be increased as mentioned above. Then, etching is an acute problem. There is a need for an antireflective film having a high selective ratio of etching relative to resist, that is, a high etching speed.
Studies have been made on light-absorbing agents for imparting an optimum absorption coefficient to an antireflective film. Anthracene and phenyl type agents are proposed for the KrF and ArF systems, respectively. However, they are also substituent groups having high dry etching resistance as described previously. Even when a polymer backbone having such organic groups as pendants is formulated as a polymer having low etching resistance such as an acrylic resin, a practical limit exists. On the other hand, silicon-containing materials are generally known to have a high etching rate under etching conditions using fluorocarbon gases and provide a high selective ratio relative to the resist. It is then believed that the use of a silicon-containing antireflective film brings about a drastically increased selective ratio in etching. For example, JP-A 11-60735 discloses an antireflective film for KrF exposure comprising a polysilane having pendant phenyl groups, achieving a high selective ratio of etching.
The recent progress toward a higher resolution accelerates the thinning of resist film. As the thickness is reduced, the resist is required to have higher etching resistance. However, improvements in etching resistance are insufficient. One method of pattern transfer for thin film resist is a hard mask method. The hard masks under consideration are SiO films when substrates to be processed are p-Si, and SiN, W—Si and amorphous Si when substrates to be processed are SiO2 films. The hard masks are disrupted in some cases and peeled in other cases. Particularly when the underlay is an insulating film such as SiO2 film, the W—Si or amorphous Si film which is a good conduction film must be peeled. The SiN film which is an insulating film need not be peeled in some cases. However, the SiN film, whose constituent elements are similar to those of SiO2 film, has a drawback that the selective ratio of etching which is an essential function of hard mask is low. Also, a hard mask in the form of a SiON film having the additional function of antireflective film was proposed in SPIE 2000, Vol. 4226, p. 93.
There have been proposed a number of pattern forming processes using silicon-containing polymers as the underlying film below the resist. For example, Japanese Patent No. 3,118,887 and JP-A 2000-356854 disclose a three-layer process involving forming an organic film on a substrate, spin coating silica glass thereon, transferring a resist pattern to the silica glass layer, effecting oxygen gas etching for transferring the pattern to the organic film layer, and finally processing the substrate. JP-A 5-27444, JP-A 6-138664, JP-A 2001-53068, JP-A 2001-92122 and JP-A 2001-343752 disclose silica glass layers serving as an antireflective film as well and silsesquioxane polymer materials. U.S. Pat. No. 6,420,088 discloses a silsesquioxane polymer. JP-A 2003-502449 discloses a material based on a spin-on-glass material, serving as both an antireflective film and a hard mask. However, all these silicon-containing polymers are less storage stable and suffer from the fatal defect that film thickness varies on actual use.
As an example of silica based film, in connection with the dual-damascene manufacture by via-first process, Richard Spear et al. proposed spin-on-glass materials as the antireflective coating/filling material in JP-A 2003-502449, U.S. Pat. Nos. 6,268,457 and 6,506,497. Also a spin-on-glass material having no antireflective effect was proposed as the low-dielectric constant film-forming filling material. The spin-on-glass materials have a high structural similarity to the silica base low-dielectric constant film and raise no problem in pattern shape during fluorocarbon gas dry etching, but fail to establish a selectivity during wet etching and are difficult to control the shape after stripping.
On the other hand, when organic materials are used as the filling material, they tend to generate shape abnormalities in proximity to the interface between the organic film and the low-dielectric constant film during fluorocarbon gas dry etching for low-dielectric constant material processing after the oxygen gas dry etching step.
One of the performance requirements for the antireflective film is to eliminate intermixing with the resist and diffusion of low-molecular-weight components into the resist layer, as discussed in Proc. SPIE Vol. 1, 2195, pp. 225-229 (1994). One effective means taken to prevent intermixing and diffusion is by baking an antireflective film after spin coating for inducing thermal crosslinkage. A resist pattern on the antireflective film or the resist underlying film is desired to have a perpendicular shape without footing or undercut. This is because the footed shape introduces a difference of dimensional conversion after etching of the antireflective film, and the undercut shape causes the resist pattern to collapse after development.
It is reported in Proc. SPIE Vol. 3678, pp. 241-250 (1999) that acid-assisted crosslinkage is effective for restraining the positive resist from footing. The method of adding a crosslinking agent and crosslinking with the aid of acid is important for antireflective materials. JP-A 5-27444 and JP-A 2001-92122 describe that the addition of crosslinking agents is effective.
JP-A 2001-354904 describes a process of preparing a porous film-forming composition in the presence of tetraalkylammonium hydroxide. This composition is to form a silica base interlayer dielectric layer. In Example, the composition is fired at a temperature of 450° C. which is above the decomposition temperature of organic groups. The resulting inorganic film can have a good selective ratio of etching relative to an organic resist film, but the film formation by firing at such a high temperature as to decompose organic groups and relying on non-organic crosslinkage fails to achieve optical and acid-diffusion-preventing functions as intended in the present invention.
The recent demands for semiconductor integrated circuits having higher degrees of integration and higher operating speeds require an interlayer insulating layer having a lower dielectric constant in order to reduce the interconnection capacitance. As the insulating layer having a low dielectric constant, porous films are now under study instead of conventional silicon oxide films. A conventional inorganic silicon film formed on such a porous film cannot be selectively removed because of similar properties.
The inventors discovered in Japanese Patent Application No. 2003-157807 a resin composed mainly of a silicon base material in which acid-catalyzed crosslinking of organofunctional groups provides satisfactory lithographic properties, a satisfactory etching selectivity relative to an organic material, and storage stability. A film formed after crosslinking of organofunctional groups has a higher dry etching resistance than a porous dielectric film, and is difficult to remove by wet etching without causing damage to the porous dielectric film.